1. Field of the Invention
The present invention relates to a nitride semiconductor laser element and to a method for manufacturing this element, and more particularly relates to a nitride semiconductor laser element having a ridge waveguide structure, and to a method for manufacturing this element.
2. Description of the Prior Art
There is a growing need for semiconductor laser elements featuring a nitride semiconductor formed from a compound semiconductor of InxAlyGa1-x-yN (0≦x, 0≦y, 0≦x+y≦1) to be utilized in optical disk systems that allow large-capacity and high-density information recording and reproduction, such as next-generation DVDs, and to be utilized in personal computers and other such electronic devices. Accordingly, a great deal of research has gone into semiconductor laser elements that make use of nitride semiconductors.
In particular, various researches as been aimed at laser element structures, the result of which is that a structure which allows favorable lateral mode control, a structure that affords reduced power consumption, higher output, better reliability, a smaller size, a longer service life, and so forth, and other such structures have been proposed. Of these, the structures that are viewed as the most promising are those having a ridge waveguide structure, and this ridge waveguide structure has even been employed in nitride semiconductor laser elements, which were the world's first to appear on the market.
Also, further threshold current reductions are needed for nitride semiconductor laser elements. Reducing the threshold of a laser element requires that the horizontal-lateral mode be stabilized.
Usually, with a laser element featuring a ridge waveguide structure, optical confinement in the lateral mode must be carried out with good control and reproducibility in order to stabilize the horizontal-lateral mode, and a known way of accomplishing this is to use a protective film with a low refractive index as an embedding film.
For example, there has been proposed a compound semiconductor laser capable of lateral mode control, in which an embedding layer composed of a dielectric film is formed on both sides of a ridge component (see Japanese Laid-Open Patent Application H10-270792, for example).
There has also been proposed a laser element comprising a ridge composed of a second cladding layer of a second conductivity type and a cap layer of a second conductivity type, a dielectric film formed on the side faces of the ridge except for the very top part of the ridge, and an electrode metal layer that covers the ridge, wherein there is a cavity between the dielectric film or electrode metal layer and the subsequently formed thick-film electrode, in the upper face portion of the ridge (see Japanese Laid-Open Patent Application 2005-166718, for example). Also, the formation of a gap at the ridge of a laser element has been proposed (Japanese Laid-Open Patent Application 2005-64262).
However, the optical confinement of a laser element in which a protective film with a low refractive index is formed on the side faces of the ridge and on the surface of a nitride semiconductor layer on both sides of the ridge is dependent on the material of this protective film.
Also, it was difficult to control optical confinement in the horizontal-lateral mode because it affected the adhesion and thickness of the protective film formed on the semiconductor laser on both sides of the ridge. Optical confinement in horizontal lateral mode requires that the refractive index difference from that of the nitride semiconductor layer be made equal on both sides of the ridge, rather than just employing a protective film with a low refractive index and providing this refractive index difference.
With the structure described in Japanese Laid-Open Patent Application H10-270792, for example, the embedding layer is formed in a thickness that is equal to the height of the upper surface of the ridge from the exposed part of the semiconductor laser, so a difference in the coefficients of thermal expansion between the semiconductor laser and the embedding layer may result in interfacial separation between the embedding layer and the semiconductor laser, which leads to current leakage. There is also the danger that cracks will develop in the embedding layer and that current will leak from these cracks.
With the semiconductor laser element described in Japanese Laid-Open Patent Application 2005-166718, there is an cavity on the outside of the dielectric film formed on the side faces of the ridge, so lateral mode control is unlikely to be possible. Furthermore, since this semiconductor laser element is a GaAs-based semiconductor element, it has an eave-like shape, but with a nitride semiconductor the ridge shape tends to be difficult to form in an eave shape.
Also, with a configuration in which a gap is enclosed by a dielectric film, a large current is required, and with a GaN-based semiconductor element that is prone to generating heat, its operation over time results in stress being applied between the gap and the dielectric film. Consequently, separation tends to occur at the interface between the two, the gap itself becomes unstable, and this causes the operating current to be unstable, making it impossible to control optical confinement.